Patent specification EP 0 449 208 A2 discloses jib sections which are more or less round or, instead of a circle, are based on an upstanding ellipsis. Jibs made from these cross-sections have a relatively low rigidity about the vertical and horizontal axis, a relatively low torsion resistance and a low resistance to twisting. Sections with half-box shaped or, alternatively, trapezoidal top shells are also described. The straight legs of the top and bottom shell portions lie in one plane. With this shape, the two lateral and the top segments are highly susceptible to buckling.
Patent specification EP 0 668 238 A1 discloses a jib section, comprising a half-box shaped segment and a rounded bottom segment. The rounded bottom segment has at least one flat wall portion. The straight legs of the top half-box shaped segment lie parallel with the vertical axis of symmetry of the section and are joined to the bottom legs. Both the top and the bottom jib segments are highly susceptible to instability, especially during displacement, in the regions of the overlap and at points where force is transmitted.
Document DE 200 04 016 U1 discloses a jib cross-section with a bottom rounded part and a top half-box shaped part, the oppositely facing legs of which are welded to one another. The top part has the shape of an equal-sided trapezium without a longer base line. Extending parallel with the vertical axis of symmetry, the legs of the bottom section abut with the legs of the top profiled part forming an angle. In order to reduce the risk of buckling in the thinner top shell, the thicker bottom shell must terminate far beyond the neutral zone of the cross-section. Furthermore, the overall cross-section is less rigid due to the fact that the straight web walls are drawn in. A jib of this design is higher in weight and has a greater overall deformation.
A telescopic jib with a bottom segment comprising several adjoining, outwardly curved shell segments is disclosed in patent specifications DE 196 24 312 A1 and EP 0 814 050 B1. The bending strength, torsion resistance and efficient transmission of load in the bottom part, which is subjected to pressure, is significantly improved. This cross-section also has a top half-box shaped segment in which the straight legs lying in one plane are welded to one another.
Utility model DE 202 20 121 U1 describes a jib cross-section with a top shell comprising two outwardly curved shell segments, and the straight leg ends of the top and bottom cross-section parts extend parallel with the vertical axis of symmetry and are welded to one another. With this known profile, not only is the bottom part designed to withstand pressure stresses, resistance to buckling is also increased in the top part.
With the known telescopic jibs, attempts have therefore been made to optimize the top shell or the bottom shell, depending on the type of crane. There is no perceptible standard overall concept. Crane jibs are primarily subjected to bending stress perpendicular to and transversely to the luffing plane. Due to wind, pivoting, etc., the jib is subject to high lateral loads. If the jib is additionally anchored or pre-tensioned, the entire top and bottom part is subjected to pressure stresses. In standard jib cross-sections, the top cross-sectional part is primarily made from flat sheets and the outer corners are rounded. All of these cross-sections in which the top segment is of a half-box shaped design are highly susceptible to buckling. The bottom cross-sectional part differs significantly from the top part in terms of its shape and the shell segments and/or multiple edges are intended to improve resistance to buckling and load transmission.